MicroRNAs (miRNAs) are small non-coding endogenous RNA molecules that are thought to be involved in the pathogenesis of pulmonary arterial hypertension (PAH) though their exact roles are not known. We found that smooth muscle cell (SMC)-specific knockout of miR-17~92 in mice attenuated hypoxia-induced pulmonary hypertension (PH) and reconstitution of miR-17~92 restored it, indicating an important role for miR-17~92 in pathogenesis of PH. We identified that miR-17~92 directly targets prolyl hydroxylase 2 (PHD2) and PDZ and LIM domain 5 (PDLIM5) proteins. Suppression of miR-17~92 induced PHD2 expression and inhibited HIF activity, induced PDLIM5 expression and decreased TGF-/Smad signaling and expression of SMC markers, all of which attenuated PH. SMC-specific knockout of PHD2 and PDLIM5 enhanced hypoxia-induced pulmonary artery remodeling whereas overexpression of PDLIM5 inhibited hypoxia-induced PH. These results indicate that miR-17~92 modulates PH by regulating the expression of PHD2 and PDLIM5. In PASMC in-vitro and mouse lungs in-vivo, chronic hypoxia resulted in a biphasic expression of miR-17~92: an early increase followed by a decrease in expression. We found that miR-17~92 expression was reduced in PASMC isolated from PAH patients and that this reduction in miR-17~92 accounted for the de-differentiated phenotype of the IPAH-PASMC. We speculate that up regulation of miR-17~92 may be a common initial event in the pathogenesis of both human and experimental PH and that the late decrease in miR-17~92 expression may be an adaptive mechanism to inhibit further progression of PH. Our hypothesis is that miR-17~92 initiates the pathogenesis of PAH by: 1) directly suppressing PHD2 to activate the HIF pathway; 2) directly suppressing PDLIM5 to activate the TGF-/Smad2/3 pathway. The biphasic nature of miR-17~92 expression in chronic hypoxia is a novel finding and we will investigate its significance and the mechanisms involved. In Specific Aim 1, we will determine the molecular mechanisms by which miR-17~92 and PHD2 regulate hypoxia- induced PH. We will investigate whether PHD2 is a novel direct target of miR-17~92 and the molecular mechanisms by which miR-17~92 and PHD2 regulate HIF activity and PH. Specific Aim 2 is to determine the molecular mechanisms by which miR-17~92 and PDLIM5 regulate hypoxia-induced vascular remodeling and PH. We will investigate whether PDLIM5 is a novel direct target of miR-17~92 and the molecular mechanisms by which PDLIM5 negatively regulates TGF-/Smad signaling and inhibits the progression of PH. In Specific Aim 3, we will determine the molecular mechanisms underlining the biphasic expression of miR-17~92 and its implication in PH progression. We will investigate the roles of HIF and E2F1 in up regulation of miR-17~92 in the early phase of chronic hypoxia, the role of p53 in inhibition of miR-17~92 in the late phase of chronic hypoxia, and whether knockout of miR-17~92, PHD2, and PDLIM5 in the late stage of hypoxia diminishes or accentuates PH in mice.